Method for the manufacture of cast iron parts having internally arranged friction bearing surfaces

ABSTRACT

Method for the manufacture of gray cast iron parts having internally arranged friction bearing surfaces, especially rockerarms for internal combustion engines, which are provided with a bearing bore produced by a core placed into the mold. The core can take the form of a thin-walled steel bushing which is removed from the bearing bore after the cast iron part has cooled and is removed from the mold. The core can be removed by either turning-out or pressing-out, and after removal of the core, the bore or at least a portion of the bore can be ground to the desired dimensions. Prior to the casting process, the core can be subjected to either heating or cooling, and during the casting process, a cooling medium stream can be passed through the core.

0 United States Patent 1 1 [111 3,760,863 Weyersberg Sept. 25, 1973 METHOD FOR THE MANUFACTURE OF 1,391,675 9/1921 Emery 164/126 CAST IRON PARTS HAVING INTERNALLY w ar ow ARRANGED FRICTION BEARING 2,304,879 12/1942 Brazil 164/30 SURFACES 3,483,916 12/1969 Hard 164/128 x Inventor: Ernest Rudolf Weyersberg,

Stuttgart-Bad Cannstatt, Germany Assignee: Firma Dr. -lng. h. c. F. Porsche KG,

Germany Primary ExaminerJ. Spencer Overholser Assistant Examiner.lohn S. Brown Att0rneyCraig, Antonelli, Stewart and l-lill [57] ABSTRACT Method for the manufacture of gray cast iron parts having internally arranged friction bearing surfaces, especially rockerarms for internal combustion engines, which are provided with a bearing bore produced by a core placed into the mold. The core can take the form of a thin-walled steel bushing which is removed from the bearing bore after the cast iron part has cooled and is removed from the mold. The core can be removed by either turning-out or pressing-out, and after removal of the core, the bore or at least a portion of the bore can be ground to the desired dimensions. Prior to the casting process, the core can be subjected to either heating or cooling, and during the casting process, a cooling medium stream can be passed through the core.

14 Claims, 1 Drawing Figure PAIENIEB W 3.760.863

/nve/7f0r: ERNST RUDOLF WEYERSBERG' BY Owl QM 44191 ATTORNEYS METHOD FOR THE MANUFACTURE OF CAST IRON PARTS HAVING INTERNALLY ARRANGED FRICTION BEARING SURFACES BACKGROUND OF THE INVENTION The present invention relates to a method for the manufacture of cast iron parts having internally arranged friction bearing surfaces, and more particularly, to the manufacture of gray cast iron parts for rockerarms in internal combustion engines which are provided with a bearing bore which is produced by a core placed into the mold;

It is known to create local surface zones of greater hardness in cast iron parts by influencing the cooling speed. With gray cast iron produced by sand casting, chilling bodies which are good heat conductors are placed into the mold to influence the cooling speed of the cast iron at the surface zones, whereby the depth of the surface zones influenced by the greater cooling speed may be adjusted through corresponding dimensioning of the bodies. Depending upon the composition, especially with regard to the alloying constituents C and Si of the cast iron, varied texture structures having different mechanical characteristics in the surface zone may be further obtained through variation of the cooling speed, whereby the attainable texture structure for a given cast iron composition depends essentially on the intensity of under cooling.

For mass production of cast iron parts of definite texture structure in the surface zones, permanent steel forms are used which, because their wall thickness cannot be arbitrarily reduced for reasons of strength, are provided with a covering of poor or non-heat conducting material for reducing their heat conducting ability.

The aforementioned means for influencing the texture formation in the surface zone by means of cooling speed are applied only for outer arranged surfaces of cast iron parts. In this connection, see High-Grade Cast Iran by E. Piwowarski (Springer, Berlin 1951). Among other things, it was thought impossible to find an advantageous solution for internal sliding surfaces. Insofar as internal surfacesforcast iron parts were concerned, a distinct surface hardness was necessary. Thus, they were regularly subjected to after-treatment such as hardening and subsequent tempering or nitriding, chromium plating or hardening and the like. The friction bearing surface, expecially for rockerarms of internal combustion engines, operatively cooperating with the rockerarm shaft was formed subsequently by insertion of a bushing into the bearing eye of the rockerarm. Insertion of a bushing into the bearing eye of the rockerarm is, however, expensive since the bearing eye must be machine finished prior to pressing the bushing into it.

SUMMARY OF THE INVENTION It is the aim of the present invention to overcome the problems and disadvantages encountered in the prior art.

Further, it is a main object of the present invention to effect the manufacture of a ledeboric sliding layer in bores or recesses of gray cast iron parts.

The foregoing problems and disadvantages are solved in accordance with the present invention by providing a method through which a favorable texture structure is obtained with simple means which are suitable for a friction bearing surface without extensive aftertreatment for cast iron parts of the above-mentioned kind, especially for rockerarms provided with a bearing bore for internal combustion engines.

The foregoing problems are also solved in accordance with the present invention by forming the core of a thin-wall steel bushing which is inserted into the casting mold and, after the casting is removed from the mold, the bushing is removed from the bore and is finally ground to the proper dimensions or, at least, the portions of the bore which form the friction bearing surfaces.

In accordance with the present invention, the steel bushing may be removed from the bore by means of turning, after removal of the casting from the mold. Especially with continuous or through bores, recesses or the like, the steel bushing may also be pressed out. Through proper dimensioning of the wall thickness of the steel bushing, it is possible to adjust the depth of the surface zone which is influenced by the higher cooling speed.

The present invention further provides that the steel bushing is preheated prior to the casting process in order to obtain a definite texture formation on the surface zones through one cooling speed or its temperature magnitude, respectively. By the use of cast iron with about 3 or 4 percent C and less than 2.5 percent Si, the bushing is so pre-heated that a cooling speed of about 500 C/min. is attained whereby a ledeboritic texture is formed at the surface zones which shows satisfactory running characteristics with sufficient hardne'ss.

Through an increase in the cooling speed and a possible concurrent decrease of Si as an alloying constituent, the dissociation of carbide may be decreased so that a harder texture structure is provided at the surface zone. Conversely, a decrease in the cooling speed and a possible simultaneous increase of Si as an alloying constituent leads to an increase in the dissociation of carbide and therewith to a lesser hard texture structure in the surface zone.

For increasing the heat leak-off capacity of the steel bushing, the method of the present invention further provides for, passinga cooling stream through the bushing whereby a constant heat leak-off through the steelbushing is attained for an arbitrary time, and by this means the depth of the surface zone influenced by the increased heat leak-off speed may be arbitrarily varied.

In accordance with a still further feature of the method of the present invention, a number of cast parts can be placed together in one mold for the manufacture of smaller continuous bores or recesses, respectively in cast iron parts, and in place of providing single steel bushings for each part, a common steel tube passing through a number of casting parts may be used. By this means, the passing of a cooling medium stream through the steel bushings of a number of casting parts can be simplified, aside from the forming-in process.

Finally, the method of the present invention provides still a further measure for influencing the cooling speed by covering the steel bushings or steel tube with a layer of poor or non-heat conducting material.

The method of the present invention results in several advantages. By the use of a heat conducting steel bushing in place of a core of non-heat conducting material, a higher cooling speed of the cast iron and, thereby, a greater hardness of the surface is obtained in the surface zones of bores or recesses of cast iron parts without creating more than an insignificant distortion of the recess, so that the desired friction surface or sliding surface may be produced through a simple mechanical treatment of the surface such as grinding. Through the creation of a ledeboric texture structure with adjustment of the cooling speed in each case for a higher or lower share of non-dissociating carbide and therewith a greater or lesser hardness, a sufficient hardness as well as a satisfactory running characteristic of the surface is obtained. By the use of the method of the present invention, for producing, for example, a friction bearing surface in the bearing eye of a rockerarm for an internal combustion engine, either the manufacturing steps for the turning out of the bearing eye and the assembly of the bushing as well as the bushing itself or the working steps for a metallurgical treatment such as hardening, nitriding and the like may be saved in constrast to the conventional methods used up to this time.

. As a further advantage of. the method in accordance with the present invention, it can be mentioned that through corresponding adjustment of the depth of the surface zone provided with a special texture structure as well as finishing tolerances and also the expected bearing loads of the friction bearing may be accounted for.

BRIEF DESCRIPTION OF THE DRAWING These and further features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawing which shows, for purposes of illustration only, an embodiment in accordance with the present invention and wherein:

the sole FIGURE is a cross-sectional view of a rockerarm for an internal combustion engine manufactured in accordance with the method of the present invention.

DETAILED DESCRIPTION OF THE DRAWING Referring now to the sole FIGURE, the rockerarm 1 is made of gray cast iron and consists essentially of a first lever arm 2 operatively connected with a cam shaft and a second lever 3 operatively cooperating with a valve stem as well as a bearing bore 4. At the areas of the lever arms 2 and 3 which operatively cooperate with the cam shaft and valve stem, surface zones 5 and 6 consist of a hard texture structure and are provided through insertion of corresponding ingot molds into a casting mold.

For the casting process, a surface zone of l to 2 mm. depth having a ledeboritic texture structure 7 is created at the same time along the friction bearing or sliding bearing surface 4' of the bearing bore 4 over which the rockerarm l is swingably supported on the rockerarm shaft. The sole FIGURE shows a steel bushing 8 insertedinto the casting mold for the creation of the ledeboritic texture structure 7. As a result of its heat conductivity, the steel bushing 8 is subject to a higher cooling speed of the cast iron at the surface zones along the friction bearing surface 4' of the bearing bore 4 and releases thereby the formation of a ledeboric texture structure.

After removal of the rockerarm 1 from the mold, the steel bushing 8 is removed from the bearing bore 4 through a turn-out operation and the friction bearing surface 4 is ground to the proper dimensions. It can be readily seen that no further finishing operation is required.

Although I have shown and described one embodiment in accordance with the present invention, it is to be clearly understood that the same is susceptible of numerous changes and modifications as will be apparent to one skilled in the art, and I, therefore, do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the present invention.

I claim:

1. Method for the manufacture of cast iron parts, particularly gray cast iron parts having about 3-4 percent C and less than 2.5 percent Si, and having internally arranged friction bearing surfaces, especially rockerarms for internal combustion engines having a bearing bore, comprising: inserting a core of a thin-walled steel bushing into a casting mold, casting the cast iron part in the casting mold, forming a bearing bore in the cast iron part by meaans of the core including cooling of the portion of the cast iron part adjacent the core at a predetermined starting rate of approximately 500C/min., removing the cast iron part from the casting mold, and removing the core from the bearing bore.

2. Method according to claim 1, wherein the step of removing the core includes pressing-out.

3. Method according to claim 1, further including subjecting the core to a temperature change process prior to casting of the part.

4. Method according to claim 3, wherein the temperature change process is cooling.

5. Method according to claim 3, wherein the temperature change process is heating.

6. Method according to claim 1, further including passing a cooling medium stream through the core during casting and cooling of the part.

7. Method according to claim 1, including the step of covering the core with a layer of material having low heat conductivity. I

8. Method according to claim 1, including forming a plurality of cast iron parts Simultaneously in one mold, wherein the cast iron parts form a continuous bore, and passing a single core through the bores of all the cast iron parts in lieu of individual cores for each bore.

9. Method according to claim 8, including subjecting the core to a temperature change process.

10. Method according to claim 1, including the step of cooling the portion of the cast iron part adjacent the core at the predetermined starting rate and cooling the other portions of the cast iron part at a different cooling rate.

11. Method according to claim 10, including dimensioning of the wall thickness of the core for assuring the cooling of the portion of the cast iron part adjacent the core at the predetermined starting rate.

12. Method according to claim 1, including dimensioning of the wall thickness of the core for assuring the cooling of the portion of the cast iron part adjacent the core at the predetermined starting rate.

13. Method according to claim 1, wherein the bearing bore is provided with a hardened surface layer adjacent the core, and including the step of regulating the cooling rate to vary the thickness of the hardened surface layer.

14. Method according to claim 1, wherein the bearing bore is provided with a hardened surface layer adjacent the core, and including the step of dimensioning the wall thickness of the core in accordance with a desired thickness of the hardened surface layer.

t t i i 

1. Method for the manufacture of cast iron parts, particularly gray cast iron parts having about 3-4 percent C and less than 2.5 percent Si, and having internally arranged friction bearing surfaces, especially rockerarms for internal combustion engines having a bearing bore, comprising: inserting a core of a thinwalled steel bushing into a casting mold, casting the cast iron part in the casting mold, forming a bearing bore in the cast iron part by meaans of the core including cooling of the portion of the cast iron part adjacent the core at a predetermined starting rate of approximately 500*C/min., removing the cast iron part from the casting mold, and removing the core from the bearing bore.
 2. Method according to claim 1, wherein the step of removing the core includes pressing-out.
 3. Method according to claim 1, further including subjecting the core to a temperature change process prior to casting of the part.
 4. Method according to claim 3, wherein the temperature change process is cooling.
 5. Method according to claim 3, wherein the temperature change process is heating.
 6. Method according to claim 1, further including passing a cooling medium stream through the core during casting and cooling of the part.
 7. Method according to claim 1, including the step of covering the core with a layer of material having low heat conductivity.
 8. Method according to claim 1, including forming a plurality of cast iron parts Simultaneously in one mold, wherein the cast iron parts form a continuous bore, and passing a single core through the bores of all the cast iron parts in lieu of individual cores for each bore.
 9. Method according to claim 8, including subjecting the core to a temperature change process.
 10. Method according to claim 1, including the step of cooling the portion of the cast iron part adjacent the core at the predetermined starting rate and cooling the other portions of the cast iron part at a different cooling rate.
 11. Method according to claim 10, including dimensioning of the wall thickness of the core for assuring the cooling of the portion of the cast iron part adjacent the core at the predetermined starting rate.
 12. Method according to claim 1, including dimensioning of the wall thickness of the core for assuring the cooling of the portion of the cast iron part adjacent the core at the predetermined starting rate.
 13. Method according to claim 1, wherein the bearing bore is provided with a hardened surface layer adjacent the core, and including the step of regulating the cooling rate to vary the thickness of the hardened surface layer.
 14. Method according to claim 1, wherein the bearing bore is provided with a hardened surface layer adjacent the core, and including the step of dimensioning the wall thickness of the core in accordance with a desired thickness of the hardened surface layer. 